Adaptive microphonics noise cancellation

ABSTRACT

Systems and methods are provided for compensating for mechanical acceleration at a reference oscillator. A reference oscillator provides an oscillator output signal and an accelerometer on a same platform as the reference oscillator, such that mechanical acceleration at the reference oscillator is detected at the accelerometer to produce a measured acceleration. A filter assembly, having an associated set of filter weights, receives the measured acceleration from the accelerometer and provides a tuning control signal responsive to the measured acceleration to a frequency reference associated with the system. An adaptive weighting component receives the oscillator output signal of the reference oscillator and an external signal that is provided from a source external to the platform and adjusts the set of filter weights for the filter assembly based on a comparison of the external signal and the oscillator output signal.

RELATED APPLICATIONS

This application claims priority from U.S. patent application Ser. No.16/223,777, filed 18 Dec. 2018, which is incorporated herein in itsentirety.

TECHNICAL FIELD

The disclosure relates generally to the field of communications, andmore particularly to adaptive microphonics noise cancellation.

BACKGROUND

Microphonics, or microphony, describes the phenomenon wherein certaincomponents in electronic devices transform mechanical vibrations into anundesired electrical signal. Mechanical acceleration, such as vibrationor shock, can cause frequency modulation at oscillators, resulting inmicrophonics phase noise sidebands in signals. Piezoelectrical crystalscan be particularly vulnerable to this effect, and mechanical vibrationcan transiently change the resonant frequency of the crystal andintroduce significant phase noise sidebands through inadvertentfrequency modulation. This error can propagate and multiply throughoutthe system, as any oscillator phase locked to the reference oscillatorwill be affected, such as the sampling clocks for analog-to-digitalconverters and digital-to-analog converters.

SUMMARY

In accordance with one example, a system includes a reference oscillatorthat provides an oscillator output signal and an accelerometer on a sameplatform as the reference oscillator, such that mechanical accelerationat the reference oscillator is detected at the accelerometer to producea measured acceleration. A filter assembly, having an associated set offilter weights, receives the measured acceleration from theaccelerometer and provides a tuning control signal responsive to themeasured acceleration to a frequency reference associated with thesystem. An adaptive weighting component receives the oscillator outputsignal of the reference oscillator and an external signal that isprovided from a source external to the platform and adjusts the set offilter weights for the filter assembly based on a comparison of theexternal signal and the oscillator output signal.

In accordance with another example, a method is provided forcompensating for mechanical acceleration at a reference oscillator. Amechanical acceleration is detected at an accelerometer on a sameplatform as the reference oscillator to produce a measured acceleration.A tuning control signal responsive to the measured acceleration isprovided at a filter assembly having a set of filter weights. The set offilter weights for the filter assembly is adjusted based on a comparisonof an external signal that provided from a source external to theplatform and an oscillator output signal of the reference oscillator.The tuning control signal is provided to a frequency referenceassociated with the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a communications system utilizing a referenceoscillator;

FIG. 2 illustrates one example of an adaptive weighting component thatcould be used in the system of FIG. 1;

FIG. 3 illustrates one example of a communications system utilizing areference oscillator that produces an oscillator output signal;

FIG. 4 illustrates another example of a communications system utilizinga reference oscillator that produces an oscillator output signal;

FIG. 5 illustrates yet another example of a communications systemutilizing a reference oscillator that produces an oscillator outputsignal;

FIG. 6 illustrates a further example of a communications systemutilizing a reference oscillator that produces an oscillator outputsignal; and

FIG. 7 illustrates a method for compensating for mechanical accelerationat a reference oscillator.

DETAILED DESCRIPTION

Various examples of the systems and methods described herein provide anoise cancellation system that can be used to generate a tuning controlsignal that modulates the reference oscillator to cancel or minimize thenoise caused by mechanical acceleration at the reference oscillator. Tothis end, the acceleration at the location is measured and provided toan adaptive filter, comprising an associated set of weights, to generatethe tuning control signal. The weights can be adapted, at periodicintervals, according to a measured phase error (or frequency error) ofan oscillator output signal of the reference oscillator using anexternal signal provided to the system to account for changes in theresponse of the reference oscillator to acceleration. Accordingly, alower cost, non-ruggedized reference oscillator can be used without asignificant increase in microphonic noise or the expense and addedweight of a mechanical isolation structure. Further, minor variationsamong reference oscillators introduced during fabrication can becompensated for without time-consuming testing of individual units.

FIG. 1 illustrates a communications system 100 utilizing a referenceoscillator 102 that produces an oscillator output signal 103. Thereference oscillator 102 can comprise, for example, an electronicoscillator, such as a Hartley oscillator or a Colpitts oscillator, or acrystal oscillator comprising a piezoelectric crystal. Thecommunications system 100 includes an accelerometer 104 on a sameplatform 105 as the reference oscillator 102, configured such that anymechanical acceleration at the reference oscillator is detected at theaccelerometer. Accordingly, the accelerometer 104 can continuously orperiodically produce a measured acceleration 106 representing thatexperienced at the reference oscillator 102. It will be appreciated thatfor some implementations of the reference oscillator 102, the oscillatorwill have varying sensitivity to accelerations from differentdirections, and the accelerometer 104 can be implemented as a tri-axialaccelerometer that measures the acceleration along three mutuallyperpendicular axes.

The communications system 100 further includes an adaptive filterassembly 108 that receives the measured acceleration 106 from theaccelerometer and generates a tuning control signal 110 responsive tothe measured acceleration 106 according to a set of filter weights. Thetuning control signal 110 is provided to a frequency referenceassociated with the system, in this implementation, the referenceoscillator 102. It will be appreciated, however, that the frequencyreference can be another system component that utilizes the output ofthe reference oscillator 102. It will be appreciated that the filterweights represent the response of the reference oscillator 102 toacceleration, allowing the filter assembly 106 to correct the oscillatorfor perturbation caused by the measured acceleration.

In some implementations, the response of the reference oscillator 102 toacceleration will vary over time, for example, due to aging ofcomponents and changes in the operating environment. Accordingly, theadaptive filter assembly 106 can utilize adaptive weights that areadjusted over time to account for changes in the response of thereference oscillator 102. Since the response of the reference oscillator102 to acceleration, in general, varies slowly, the adaptation can beslow relative to the system, ranging, for example, between three hertzand two kilohertz. It will be appreciated, however, that theoptimization used to produce the weights will take a certain amount oftime to converge, and the adaptation must be performed with sufficientfrequency to allow the weights to converge faster that the change in theresponse at the reference oscillator 102. Initial values for the filterweights can be set to accelerate convergence of the filter 106 accordingto known characteristics of the reference oscillator 102.

The weights for the filter assembly 112 can be provided by an adaptiveweighting component 114 that receives the oscillator output signal 103and an external signal 116. The term “external signal,” as it is usedherein, refers to a signal provided from a source external to theplatform containing the reference oscillator 102. Accordingly, theexternal signal 116 is generated in a manner that is unaffected by theany acceleration experienced at the reference oscillator. The adaptiveweighting component 114 adjusts the set of filter weights 112 for thefilter assembly based on a comparison of the external signal and theoscillator output signal. The adaptive weighting component 114 can beimplemented in digital logic, for example, as a field programmable gatearray or an application specific circuit, in software on anon-transitory computer readable medium executed by an associatedprocessor, or in some combination of hardware and software. It will beappreciated that the adaptive filter assembly 106 can be provided withan initial set of weights at the time of manufacture or installation,with the adaptive filter weights 112 provided periodically to adjust forchanges in the response of the reference oscillator 102.

FIG. 2 illustrates one example of an adaptive weighting process 200incorporating an adaptive weighting component 210 that could be used inthe system of FIG. 1. The adaptive weighting component 210 comprises ademodulator 202 that determines a phase error 203, Θ_(e)(n), in theoscillator output signal 204 of the reference oscillator 205 from theoscillator output signal 204 and the external signal 206. A frequencyestimation filter 216 in the adaptive weighting component 210 calculatesan instantaneous frequency, f(n), from the determined phase error 203 inthe oscillator output signal. In one implementation, the frequencyestimation filter 216 can be a differentiator filter with a frequencyresponse, H(f)=j2πf, a phase difference filter,f(n)=Θ_(e)(n)−Θ_(e)(n−1), or any other appropriate implementation.

Respective values 222-224 for the acceleration along each axis, asmeasured at the accelerometer 104, are filtered at respective adaptivefilters 226-228 and summed, at an adder 230, to produce a tuning controlsignal 232, representing a compensation frequency, f_(c)(n) 232, whichis provided to the reference oscillator 205. Each of the respectiveadaptive filters 222-224 and the adder 230 can be implemented, forexample, as digital logic in at a digital signal processor, anapplication specific integrated circuit, or a field programmable gatearray. It will be appreciated that the adaptive filters 226-228 canrepresent portions of the filter assembly 106 illustrated in FIG. 1, andthe respective outputs 236-238 of the adaptive filters 226-228, incombination, provide the tuning signal 110. The tuning control signal232 is provided to the frequency estimation filter 216 and thecompensation frequency represented by the tuning control signal can becompared to the instantaneous frequency to produce a frequency error242, f_(e)(n). This frequency error 242 can be utilized at a weightcomputation component 244, along with the values 222-224 for theacceleration along each axis, to produce new weights for the adaptivefilters 226-228 that minimize the frequency error 242. Further, thefrequency error signal 242 can be provided to the reference oscillator205 (not shown in FIG. 2) to adjust its frequency as depicted by line110 in FIG. 1. In another embodiment, the frequency error signal 242 canbe used for digital correction of the frequency as will be discussed inFIG. 6. The weight computation component 244 can employ, for example,algorithms that minimize the mean square of the frequency error, such asthe Least Mean Square (LMS) algorithm, the Recursive Least Mean Squarealgorithm, and gradient descent algorithms.

In one example, a Least Mean Square algorithm is used, with a vector, w,of k filter coefficients for each adaptive filter 226-228, where k is apositive integer greater than 1. The measured acceleration values222-224 along each axis at a time, n, can be represented as vectors, α,including the k most recent measurements. For a time n+1, the weightsfor the filter can be calculated as:

w _(x)(n+1)=w _(x)(n)+μα_(x)(n)f _(e)(n−d)

w _(y)(n+1)=w _(y)(n)+μα_(y)(n)f _(e)(n−d)

w _(z)(n+1)=w _(z)(n)+μα_(z)(n)f _(e)(n−d)  Eq. 1

Where μ is a convergence coefficient, selected according to theimplementation, and d is a delay that is calculated to temporally alignthe frequency estimate from the received phase and the measuredacceleration to compensate for filter delays along the two signal paths.It will be appreciated that, when the acceleration measured at theaccelerometer 104 is low, for example, when a magnitude of the measuredacceleration falls below a predefined threshold value, the adaptiveweighting component 210 may stop adjusting the weights at the filters226-228 for some time to allow the acceleration vectors to populate withmeaningful values for the optimization calculation.

FIG. 3 illustrates one example of a communications system 300 utilizinga reference oscillator 302 that produces an oscillator output signal303. The oscillator output signal 303 is provided at least to each ofreceiver front end 304 and a transmitter 305. An accelerometer 306 on asame platform 307 as the reference oscillator 302, detects mechanicalacceleration at the platform 307. In the illustrated example, theaccelerometer 306 can be implemented as a tri-axial accelerometer. Anadaptive filter assembly 308 receives the measured acceleration 309 fromthe accelerometer 306 and provides a tuning control signal 310responsive to the measured acceleration to the reference oscillator 302.

A set of weights 312 for the adaptive filter assembly 308 can bedetermined at an adaptive weighting component 314. An external, cleansignal 316 is received at the receiver front-end 304 and provided to theadaptive weighting component 314, along with the measured acceleration309. It will be appreciated that the oscillator output signal 303provided to the transmitter 305 is adjusted, at an adaptive filterassembly 308, to remove the effects of acceleration local to thereference oscillator 302. Accordingly, a signal 318 transmitted by thetransmitter 305 is a “clean” signal like the external signal 316. Theadaptive weighting component 314 can determine a degree of phase errorin the oscillator output signal 303 based upon the received externalsignal 316. From this phase error, the adaptive weighting component 314determines appropriate weights for the adaptive filter assembly 308 byminimizing a square of a frequency error derived from the phase error.This can be performed periodically to account for changes to theresponse of the reference oscillator 302 to acceleration due to aging orchanges in the operating environment.

FIG. 4 illustrates another example of a communications system 400utilizing a reference oscillator 402 that produces an oscillator outputsignal 403. The oscillator output signal 403 is provided at least toeach of a receiver front end 404 and a transmitter 405. An accelerometer406 on a same platform 407 as the reference oscillator 402, detectsmechanical acceleration at the platform 407. In the illustrated example,the accelerometer 406 can be implemented as a tri-axial accelerometer.An adaptive filter assembly 408 receives the measured acceleration 409from the accelerometer and provides a tuning control signal 410responsive to the measured acceleration 409. A set of weights 412 forthe adaptive filter assembly 408 can be determined at an adaptiveweighting component 414.

An external, clean signal 416 is received at the receiver front-end 404and provided to the adaptive weighting component 414 along with theoscillator output signal 403 and the measured acceleration 409. Theadaptive weighting component 414 can estimate a phase error 415, shownas 203 in the example of FIG. 2, in the reference oscillator outputbased upon the received external signal 416, and determine appropriateweights for the adaptive filter assembly 408 by minimizing a square of afrequency error derived from the phase error 415. This can be performedperiodically to account for changes to the response of the referenceoscillator 402 to acceleration due to aging or changes in the operatingenvironment. The adaptive weighting component 414 provides the set ofweights 412 to the adaptive filter assembly 408.

It will be appreciated that adaptive filter assembly 408 onlycompensates for frequency error due to microphonics. Other phase andfrequency errors, such as Doppler, crystal drift, and scintillation, arenot compensated for at the adaptive filter assembly 408. To addressthese sources of error, the phase error 415 can be further provided to aphase locked loop (PLL) 420. The phase locked loop 420 comprises a phaselocked loop filter 422. In one implementation, the phase locked loopfilter 422 is implemented as a low pass filter that removes any unwantedhigh frequency components present in the estimated phase error. Theresulting filtered signal can be combined with the output of theadaptive filter assembly 408 at an adder 424 to provide the tuningcontrol signal for the reference oscillator 402.

FIG. 5 illustrates yet another example of a communications system 500utilizing a reference oscillator 502 that produces an oscillator outputsignal 503. The oscillator output signal 503 is provided at least toeach of a receiver 504 and a transmitter 506 operating through adiplexer 507. An accelerometer 508 on a first platform 510 with thereference oscillator 502, detects mechanical acceleration at theplatform. In the illustrated example, the accelerometer 506 can beimplemented as a tri-axial accelerometer. An adaptive filter assembly512 receives the measured acceleration 513 from the accelerometer andprovides a tuning control signal 514 responsive to the measuredacceleration. The measured acceleration is also provided to thetransmitter 506 for transmission to a second platform 520. In oneexample, the first platform 510 is a user terminal in a communicationssystem, the second platform 520 is a satellite access node, and thecommunication between the first platform and the second platform occursvia a satellite connection. Alternatively or additionally, the firstplatform 510 can be a mobile platform, for example, implemented on anautomobile, a watercraft, and aircraft, a train, or other vehicle. Itwill be appreciated, however, that other configurations of the systemare possible, for example, with one or more user terminals used tocorrect for vibration at a satellite access node or for correctionbetween two user terminals.

The implementation of FIG. 5 exploits the fact that a signal 522transmitted from the transmitter 506 on the first platform 510 willcontain any of the microphonic error induced by mechanical vibration onthe platform that has not been corrected by other means, whereascomponents located on the second platform 520 will be unaffected by anymechanical acceleration at the first platform 510. Accordingly, thetransmitted signal 522 can be received at the second platform 520 anddemodulated at an adaptive weighting component 524 associated with alocal receiver (not shown). A phase error in the signal can bedetermined during demodulation via a frequency reference (not shown)local to the second platform, and the phase error can be used at theadaptive weighting component 516, to determine appropriate weights forthe adaptive filter assembly 512 by minimizing a square of a frequencyerror derived from the phase error. The determination of the appropriateweights by the second platform also require acceleration information,which is communicated to the second platform by the first platform (notshown). The calculated weights 526 can then be transmitted to the firstplatform 510 via the receiver 504 for use at the adaptive filterassembly 512. In an alternative implementation, the adaptive weightingcomponent 516 can be distributed across the first platform 510 and thesecond platform 520. In this implementation, a value indicative of thefrequency error, such as the frequency error, the phase error, or anyother indication that can be used to determine the frequency error, isdetermined at the second platform 520 can be transmitted to the firstplatform 510 for use in computing the filter weights 526.

It will be appreciated that the exchange of the accelerometer data andthe filter weights represents overhead in the communications system. Toreduce this overhead, the rate of updates to the weights can be limited,with the weights updated either periodically or on a predetermined timeschedule. When the weights are not being updated, the most recentlyupdated value can be maintained and utilized at the adaptive filterassembly 512 to correct for mechanical acceleration at the firstplatform 510. Since the change in the response of the referenceoscillator 502 to acceleration changes slowly, gating the updatefunction is this manner allows for a savings of overhead in the systemwith a minimal loss of accuracy in the oscillator output signal.

FIG. 6 illustrates yet another example of a communications system 600utilizing a reference oscillator 602 that produces an oscillator outputsignal 603. The oscillator output signal is used to drive numericallycontrolled oscillators at least one application specific integratedcircuit (ASIC) 605, as well as an associated receiver front-end 608. Itwill be appreciated that this implementation is provided merely for thepurpose of example, and that other implementations of the numericallycontrolled oscillators can be used, such as field programmable gatearrays. An accelerometer 610 on a same platform 611 as the referenceoscillator 602, detects mechanical acceleration at the platform. In theillustrated example, the accelerometer 610 can be implemented as atri-axial accelerometer. The output 612 of the accelerometer 610 can beprovided to each ASIC 605, via a first analog-to-digital converter (ADC)613. Similarly, an external, clean signal 614 is received at thereceiver front-end 608 and provided to the ASIC 605 via a second ADC615.

An exemplary ASIC 605, containing a numerically controlled oscillator622 that provides a reference signal for an associated transmitter 623,is illustrated in detail. In the ASIC 605, an adaptive filter assembly624 receives the measured acceleration 612 from the accelerometer 610and provides a tuning control signal 625 responsive to the measuredacceleration 612. The tuning control signal 625 from the adaptive filterassembly 624 can be supplemented by an additional tuning signal from aloop filter 630 at an associated adder 626, in order to track othersources of frequency error like Doppler shifts, oscillator drifts, etc.as discussed in connection with FIG. 4. It will be appreciated that thenumerically controlled oscillator 622 can receive the digital tuningsignal, and it is thus unnecessary to convert the digital output of theadaptive filter assembly 624 to an analog signal.

Each of the external signal 614, an output 628 of the numericallycontrolled oscillator 622, and the output 612 of the accelerometer 610is provided to an adaptive weighting component 632. The adaptiveweighting component 632 includes a demodulator (not shown) thatestimates a phase error 634, shown as 203 in the example of FIG. 2, inthe numerically controlled oscillator output 634 based upon the externalsignal 614. This phase error 634 is provided to the loop filter 630 fortracking other sources of frequency error, as described previously. Theadaptive weighting component utilizes the estimated phase error, alongwith the accelerometer output 612 to estimate a frequency error anddetermine appropriate weights for the adaptive filter assembly 624 thatminimize a square of the frequency error.

In view of the foregoing structural and functional features describedabove, an example method will be better appreciated with reference toFIG. 7. While, for purposes of simplicity of explanation, the examplemethod of FIG. 7 is shown and described as executing serially, it is tobe understood and appreciated that the present examples are not limitedby the illustrated order, as some actions could in other examples occurin different orders, multiple times and/or concurrently from that shownand described herein. Moreover, it is not necessary that all describedactions be performed to implement a method

FIG. 7 illustrates an example of a method 700 for compensating formechanical acceleration at a reference oscillator. At 702, mechanicalacceleration is detected at an accelerometer on a same platform as thereference oscillator to produce a measured acceleration. At 704, atuning control signal responsive to the measured acceleration isprovided at a filter assembly having a set of filter weights. At 706,the set of filter weights for the filter assembly is adjusted based on acomparison of an external signal that provided from a source external tothe platform and an oscillator output signal. For example, a phase errorin the oscillator output signal can be determined from the externalsignal and the oscillator output signal, a frequency error can beestimated and the set of filter weights can be adjusted according to thedetermined frequency error.

It will be appreciated that determining the adjustment to the set offilter weights can be performed locally, remotely, or at a combinationof local and remote components. In one example, a signal is generatedusing the oscillator output signal is transmitted from the platform to aremote platform, and a phase error in the oscillator output signal iscalculated from the external signal, which is generated at the remoteplatform, and the signal generated from the oscillator output signal.The calculated phase error is then transmitted to the platform, and theset of filter weights for the filter assembly is adjusted from thecalculated phase error at the remote platform. In one implementation,the set of filter weights for the filter assembly is determined onlyperiodically, such that the accelerometer and the filter are active attimes when the set of filter weights is not being determined.

At 708, the tuning control signal is provided to a frequency referenceassociated with the system to correct for errors caused by the detectedacceleration. In one implementation, the frequency reference is thereference oscillator. In another implementation, the frequency referenceis at least one numerically controller oscillator driven by theoscillator output signal. It will be appreciated that the tuning controlsignal can correct for errors other than that caused by the mechanicalacceleration. In one implementation, a correction value can becalculated at a phase locked loop to account for additional sources ofphase and frequency error, and the correction value can be added to thetuning control signal.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

1. A system comprising: a reference oscillator that provides anoscillator output signal; an accelerometer on a same platform as thereference oscillator such that mechanical acceleration at the referenceoscillator is detected at the accelerometer to produce a measuredacceleration; a filter assembly, having an associated set of filterweights, that receives the measured acceleration from the accelerometerand provides a tuning control signal responsive to the measuredacceleration to a frequency reference associated with the system; and anadaptive weighting component that receives the oscillator output signaland an external signal that is provided from a source external to theplatform and adjusts the set of filter weights for the filter assemblybased on a comparison of the external signal and the oscillator outputsignal; wherein the reference oscillator, the accelerometer, the filterassembly and the adaptive weighing component are all implemented on theplatform, and the external signal is provided from a remote location viaa receiver at the platform.
 2. The system of claim 1, wherein thefrequency reference is the reference oscillator.
 3. The system of claim1, wherein the frequency reference is at least one numericallycontrolled oscillator driven by the oscillator output signal.
 4. Thesystem of claim 1, further comprising a phase locked loop that computesa correction value from the comparison of the external signal and theoscillator output signal and adds the correction value to the tuningcontrol signal.
 5. The system of claim 1, wherein the adaptive weightingcomponent does not adjust the set of filter weights when a magnitude ofthe measured accelerations falls below a threshold value.
 6. The systemof claim 1, wherein the oscillator output signals is provided as areference to a transmitter, such that a signal transmitted by thetransmitter is not impacted by the mechanical acceleration at thereference oscillator.
 7. The system of claim 1, wherein the platform isa first platform, the reference oscillator, the accelerometer, thefilter assembly are implemented on the first platform, and the adaptiveweighting component is implemented on a second platform, remote from thefirst platform, the first platform transmitting the oscillator outputsignal to the second platform via an associated transmitter, and thesecond platform transmitting the set of filter weights to the firstplatform via an associated second transmitter.
 8. The system of claim 1,wherein the platform is a first platform, the reference oscillator, theaccelerometer, the filter assembly are implemented on the firstplatform, and the adaptive weighting component is distributed across thefirst platform and a second platform, remote from the first platform,the first platform transmitting the oscillator output signal to thesecond platform at an associated first transceiver, and the secondplatform transmitting an indication of a frequency error in theoscillator output signal to the first platform at an associated secondtransceiver.
 9. The system of claim 1, wherein the adaptive weightingcomponent comprises a demodulator that determines a phase error in theoscillator output signal from the oscillator output signal and theexternal signal and a weight computation component that adjusts the setof filter weights based on the determined phase error in the oscillatoroutput signal.
 10. The system of claim 9, wherein the adaptive weightingcomponent comprises a frequency estimation filter that calculates aninstantaneous frequency from the determined phase error in theoscillator output signal, a compensation frequency represented by thetuning control signal, and a measured acceleration corresponding to thedetermined phase error in the oscillator output signal, the weightcompensation element determining values for the set of filtercoefficients that minimize a difference between the instantaneousfrequency and the compensation frequency.
 11. The system of claim 9,further comprising a phase locked loop that computes a correction valuefrom the phase error in the oscillator output signal and adds thecorrection value to the tuning control signal.
 12. The system of claim1, wherein the external signal is provided via a satellite associatedwith the system.
 13. The system of claim 1, wherein the accelerometer isa three-axis accelerometer that provides a measured acceleration alongeach of first, second, and third axes, and the set of filter weightsincludes a subset of filter weights for each of the first, second, andthird axes.
 14. A method for compensating for mechanical acceleration ata reference oscillator comprising: detecting mechanical acceleration atan accelerometer on a same platform as the reference oscillator toproduce a measured acceleration; providing a tuning control signalresponsive to the measured acceleration at a filter assembly having aset of filter weights, the filter assembly being implemented on the sameplatform as the reference oscillator; adjusting the set of filterweights for the filter assembly at an adaptive weighing componentimplemented on the same platform as the reference oscillator based on acomparison of an oscillator output signal of the reference oscillatorand an external signal that is provided from a remote location, externalto the platform, via a receiver on the same platform as the referenceoscillator; and providing the tuning control signal to a frequencyreference associated with the system.
 15. The method of claim 14,wherein the frequency reference is the reference oscillator.
 16. Themethod of claim 14, wherein the frequency reference is at least onenumerically controlled oscillator driven by the oscillator outputsignal.
 17. The method of claim 14, wherein determining the set offilter weights for the filter assembly comprises: transmitting a signalgenerated from the oscillator output signal from the platform to aremote platform; calculating a frequency error in the oscillator outputsignal from the external signal and the signal generated from theoscillator output signal, the external signal being generated at theremote platform; transmitting an indication of the calculated frequencyerror to the platform; and adjusting the set of filter weights for thefilter assembly from the calculated phase error at the remote platform.18. The method of claim 14, further comprising: computing a correctionvalue at a phase locked loop; and adding the correction value to thetuning control signal.
 19. The method of claim 14, wherein the set offilter weights for the filter assembly is determined only periodically,such that the accelerometer and the filter are active at times when theset of filter weights is not being determined.
 20. The method of claim14, wherein adjusting the set of filter weights for the filter assemblycomprises: determining a phase error in the oscillator output signalfrom the external signal and the oscillator output signal; and adjustingthe set of filter weights from the determined phase error.